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hello,i've lurked a while here and recently purchased a fisher 400.. i am having a little trouble with it though. fisher mods were installed by seller, and it was checked out and working ok. when received it and first powered it up, the left channel was weak & distorted, the right sounded ok, but after that i'm getting no sound out of it unless i set it to FM mono and really crank the volume (tried aux w/ cd player as well), and then it's weak and distorted.
i tried rotating the 7868s and 12ax7s in the amp stage, nothing seemed to make a difference. i do have a copy of the tech manual to check voltages etc, i can't really make up my mind to fiddle with it or just send it somewhere and have it all checked out and aligned properly. any advice would be grateful, as would a recommendation for a good place to send it for service (i'm outside of boston, thinking of East Coast Approved Audio or Antique Radio Restoration and Repair, both in Connecticut).
thanks very much and happy holidays,
matt
Follow Ups:
hi,well, i have to sheepishly admit that i am to blame for the mysterious behavior of this receiver.
after not really getting anywhere last night, something told me to look more closely at the speaker connections one more time. sure enough, i had moved the output wire taps to the 16 ohm connection, and the end of each 'spade' (where it crimps the wire) is just touching the rivets that hold the terminal block to the chassis. so i was grounding the speaker outputs, hence no sound. (and i remember thinking first off 'whenever i've had mysterious/weird problems like this, it's usually because i'm forgetting something or connecting something wrong in a basic, stupid way.' well, it happened again.)
now a cd on the aux connection sounds normal. the FM section still doesn't sound right. one channel stronger, hardly any sound in stereo, better in mono. the tuning eye does indicate when stations are tuned in. is a failing 1uF in the MPX indicative of this? or i may just be doing something else dumb.
thanks,
matt
These oversights happen to the best of us. Consider yourself lucky you didn't do real damage.I guess I should have mentioned to start with the simple stuff first.
As far as the MPX goes, start with the simple stuff first: make sure you've got good tubes in the MPX and all the sockets are clean and tight. Retubing the MPX - if needed - will not affect the alignment. A little D5 spray on the selector switch is never a bad idea. If the 1 uf 'lytic is still original, I'd replace it in any event.
If the mono FM is also subpar, then your problem for both the mono and MPX may be further upstream. Again, start with the simple stuff first, such as a good antenna connection to the correct terminals.
Good luck!
. . . if you are having problems only in one channel, the circuit tracing shouldn't go too far to find the cause. Problems originating nearer the supplies tend to affect both channels. Start close to the active components in the affected channel and it should be a quicker diagnosis.Your post suggests that the unit operated normally when you took delivery but failed thereafter. But if the problems predated your possession, an issue such as AJ has pointed out involving careless mods becomes a valid concern. A properly reconditioned Fisher should be rock solid reliable for many, many years. Typically, 95% of these instruments never need any attention, ever. It's that other 5% of the circuit that wears out and often gets botched up in service. I'd carefully inspect all the "mods" done. Pay special attention to all service soldering and redo every joint that may be the least bit suspect. There are a lot of hackers out there, including some who are paid professionals at it.
Good luck!
thanks. i'm under the impression the unit was working properly when i purchased it. what seemed to happen was i turned it on, right channel of fm signal sounded "correct", left was weak. listening to left, it was clearly a low, distorted output. after that initial use, the only time i can hear anything coming out of it is if i select fm mono and crank the volume.i will check the 7868 cathode connections as per AJ's advice. also, the power supply caps appear to be original.. there are 2 multisection 'cans' and a paper one on top, and a paper one underneath. no burnt or 'bad' looking parts/connections. one thing i did notice is the 100K bleeder resistor from B+ to ground measures only 58K.
thank you for keeping up with this thread and for your advice. i'm thinking of checking voltages over the weekend, and may return it to the seller for a refund which he's offered. part of me wants to fix it, part of me doesn't want to deal with it.
matt
hi,with a cd player in the aux input, there is an extremely weak output i can barely hear with the volume cranked. only fm mono presents any kind of audible output.
it's my first vintage electronics purchase, i bought it thinking "great, it has the fisher doc mods already." the seller has been very helpful and does seem very knowledgable about what's going on. it'll work out one way or the other.
happy holidays,
matt
Let's frame it this way: can you configure the receiver in any input or mode where the left channel plays NORMALLY? If it works right in FM mono (or any mode), then the problem is not in the preamp, driver or output stages. If so, it could even be as simple as a dirty selector switch.
hi,i'm sorry if i'm not explaining it correctly. as far as i can tell it will not play normally in either channel in any of the positions. it doesn't work correctly in fm mono, just that in fm mono i can actually hear something out of one channel (it's low and distorted though). i will try cleaning the selector switches.
thanks,
matt
But how was it for other than FM? That is an essential question.If other sources played just fine through both channels, then the problem is definitely confined to the MPX subchassis through selector switch region. Then I'd be looking for any "de-emphasis mods". Working into the MPX subchassis requires more accomplished skills.
Depending on the quality of the mods, if they have to be redone or corrected, you may want to see about getting credit off the purchase price. This is why I never buy any "modified" equipment unless it is from a bona fide, well recognized restorer of high repute. Sometimes, the instrument was better off before the "mods".
Good luck!
Hi,This is a common error in restoring these receivers. The 7868 outputs cathode v14 or v15 left channel use the cathode pin for grounding another component. If cathode resistors have been installed on all cathodes and the other component is still connected to the cathode pin then this WILL cause this problem.
Each cathode through a 10ohm resistor to ground only no other components connected to the cathodes of the outputs. If they are they must be separately connected to ground on their own. A schematic will make this obvious.
I doubt that this unit was working without distortion after the fisher mods...This is easy to do if you are not experienced in adding the cathode resistors in this unit...
Good Luck , AJ
thank you for the advice.just to make sure i'm understanding you correctly - look for any other components connected to the 7868 cathodes - if so, move connection to seperate ground?
-matt
Start with this approach: First pull ALL the tubes. Be sure to mark the tuner IF ones for exact in order replacement. Then check the plate and interstage coupling voltages from the first preamp stage through to the outputs with the unit energized. This is the easiest way to quickly isolate the failing stage or circuit. Refer to the schematic for specified voltages. Once you have identified the failing circuit, you can then trace back to the power supply for the cause(s).Also, de-rate the chassis fuse by 50% during de-tubed testing as a precaution against power transformer overload in case there is a short, or you accidentially create one.
Do not continue to swap tubes or operate the unit fully tubed or connected to anything else. You run the risk of overloading or burning something up.
Obviously, the usual caveats and warnings apply about working around active high voltages. If you don't know what you are doing, don't attempt it. A 400 that bites can kill.
"Fisher mods" mean different things to different people. If you still have original output stage coupling caps, that would be one of the first places I'd check. These things have virtually no margin for leakage. And they DO leak.
Be sure to test all your tubes once you have located and repaired the problem, especially the outputs. It is not uncommon for a circuit decline to stress an output to the verge of failure, particularly one that is cumulative and long-standing. Tired rectifiers are an example of this.
Most problems with these instruments can be quickly diagnosed with a multimeter and a logical plan. Get back to us with your results.
Good luck!
hi,thanks for your suggestions. i'll take some measurements and see what i come up with.
matt
Hello All,
My local electronics supply store stocks a pretty good selection of caps and resistors but they are all the brand of NTE or JE. Being a newbie at this I was wondering if they are any good or should I source the Spragues and etc. from the usual sources mentioned on the forum. Thanks Arch.
NTE is a rebrander, so you don't know who actually made them.While it it always best to support the local guy, Mouser is a good alternative online source for quality parts at fair prices.
(...with identically one Fisher 400 restoration under my belt!)Is the low level/distortion heard with all inputs?
You can try injecting a line-level input into the power amp input at the "spatial expander" jumpers and perhaps isolate the problem to the pre or power amp.Problem could be coupling caps (anywhere, not just the ones to the outputs) or plate resistors, for example.
Is the (fixed) bias OK? Any of the 7868's plates glowing red?
Just out of curiosity, do you know if your 400 has selenium or silicon rectifiers? Both flavors of 400 do exist, as these receivers were made for quite a few years.
There are MANY here more expert in matters Fisher than I, but I am in Boston MetroWest and would be HAPPY to take a look at it sometime mutually convenient if you'd like. I am most assuredly NOT a professional... I do like to experience these nice old receivers vicariously, though, so always happy to help if I can. I have most of the tools of the service trade, and I even know how to use some of them :-) I also do have some scanned Fisher schematics and manual pages that may be helpful, too.
Send me an e-mail if you're interested.
all the best,
mrh
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